The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
During a combustion cycle of an engine, an air/fuel mixture is delivered through an intake valve opening into a cylinder. The air/fuel mixture is compressed and combusted to provide output torque. After combustion, a piston forces exhaust gas in the cylinder out through an exhaust valve opening and into an exhaust system. The exhaust gas may contain oxides of nitrogen (NOx) and carbon monoxide (CO).
An exhaust gas treatment system of the engine may include a selective catalytic reduction (SCR) catalyst to reduce nitrogen oxides (NOx). A SCR catalyst converts the NOx into nitrogen N2 and water H2O. Reductant may be added to exhaust gas upstream from the SCR catalyst. For example only, the reductant may include anhydrous ammonia, aqueous ammonia or urea.
An exhaust diagnostic system may include a SCR catalyst monitor that monitors conversion efficiency of the SCR catalyst. The conversion efficiency refers to a rate at which the SCR catalyst reduces NOx in a certain period of time. The efficiency of a SCR catalyst may decrease over time and based on SCR catalyst usage. The SCR catalyst monitor may indicate when the conversion efficiency is less than a predetermined threshold. In some vehicles, an engine control module may limit vehicle speed and/or engine speed when conversion efficiency is less than a predetermined threshold.